The present invention relates generally to the field of printers and more particularly to the field of color impact printers for producing color characters and images from video type signals or data processing computer output.
Printers for use in the data processing field commonly are either ink jet or impact types, each type having particular advantages and disadvantages. Ink jet printer "spray" a stream of fine ink droplets towards a printing media and form ink dot printing patterns thereon by deflecting the airborne ink droplets with an electric field.
A serious disadvantage of ink jet printers is the difficulty in achieving reliable operation. Because of the required small size of the ink droplets, the ink jets and associated capillary passage ways are easily clogged. In addition, the flow of ink in such printers is very sensitive to ambient temperature and humidity, as well as to shock and vibration. Ink jet printers also typically require use of special printing media formulated to be compatible with the characteristics of the ink used to prevent excessive ink droplet smearing or ink diffusion into the media.
Color ink jet printers having several independently controlled ink jets, each of a different color, have recently been made commercially available, although at substantial cost. Such printers ordinarily achieve full color printing from the several (usually four) ink jets so closely together at each printing position that the human eye perceives only a single color combination of the different deposited droplets. Alternatively, partial or complete ink droplet overlaying techniques may be used to obtain the desired color combination at each print spot.
Although being capable of providing printing in good color directly from electronically formatted information, multi-jet color ink jet printers have severe problems. This is principally because the heretofore mentioned problems associated with ink jet printers typically multiply as ink jets are added for full color printing. As a consequence, ink jet color printers tend to be relatively unreliable, requiring frequent and expensive maintenance in order to maintain good color quality. These problems, as well as high initial costs, generally limit usefulness of currently available ink jet color printers to those relatively few applications in which printing costs are relatively unimportant.
Since ink jet color printers are generally not practical in low cost applications or those in which ambient conditions cannot easily be controlled, a substantial need exists for alternative types of color printers.
Impact printers which use rotatable balls or "daisy wheels" to print an entire alphanumeric character or symbol with a single printing stroke such as typewriters and the like, are capable of good quality printed text necessary for most printed communication purposes, at moderate speeds. However, such printers are of little practical value for producing computer graphics as is frequently necessary. Additionally, mechanical complexity causes such printers to be relatively expensive to purchase and maintain.
A less common type impact printer utilizes ball point pens which are driven against a print media. This type of printer is also usually unreliable because of ink flow interruptions and overflow. And, since rolling contact is normally required to transfer ink from the pen onto the printing media, such printers are limited to line drawing applications.
Impact printers having an array of closely spaced, individually actuable impact printing pins, on the other hand are relatively inexpensive high speed printers. Selectively actuating different printing pins in the array, enables the forming of virtually any alphanumeric character or symbol. Although such pin matrix printers, which print the desired characters and symbols as a series of unconnected dots or line segments do not provide the printed word quality of ball or "daisy wheel" impact printers, they are much more versatile than whole character printers, since "dot" matrix printouts of charts, graphs and images are enabled. This is a substantial advantage for data processing and video signal applications in which the desired visually reproduced output is required to be in other than text form.
Typically, color impact printers have used a single impact print head, and a single printing ribbon having several different transverse or longitudinal colored ink zones. After a first printing "pass" on either a line-by-line or page-by-page basis, in which a first colored ink zone of the ribbon is positioned between the print head and the printing media, a second colored ink zone of the ribbon is positioned between the head and media. A second printing "pass" is then made, the procedure being repeated until all the colors (ordinarily four) necessary to print in full color print have been printed.
Various problems are also associated with this type of color impact printing. For example, since multiple printing passes are necessary, the printing rate is relatively slow and registration of the sequential printing impacts at each printing point is difficult to achieve and maintain, particularly for other than line-by-line printing.
A further substantial problem is color contamination of various ink zones, particularly of light color areas of the printing ribbon by using only a single print head for printing all the different colors. Thus, after the ribbon has been used only a short time, and before the ink supply on the ribbon has been depleted, printing color fidelity is significantly reduced. This effect is particularly disadvantageous in those applications, such as geophysical or topographical mapping, in which the printing colors represent important information color.
Even without color contamination, inked ribbons must be replaced periodically because of ink depletion. Since such ink depletion is gradual, color impact printers may produce faded or distorted color reproduction during a length of time preceding replacement, and because of the often difficult and messy task of replacing spool held ribbon, replacement is typically deferred until the print quality is quite poor.